a) Field of the Invention
This invention relates to a method of manufacturing an integrated optical device and also to devices manufactured by the method of this invention.
b) Description of the Prior Art
Telecommunication networks using optical fibre technology are used very widely for the high speed transmission of data, throughout the developed world. Such links are able to operate with individual channel data rates of at least 2.5 Gb/s though up to 10 Gb/s can be achieved, and with wavelength division multiplexing, the capacity of an individual fibre can be increased to 40 Gb/s up to 80 Gb/s.
With such high data transmission rates, it is most important that the various components deployed throughout a telecommunication network are able to operate reliably, for most extended period of time. The most common approach to increasing the reliability of such a component is to seal the active or passive device into a package such that a perfect hermetic seal is formed, isolating the component from the ambient conditions.
The hermetic sealing of a device such as a semi-conductor laser or a detector is well-known, for example from U.S. Pat. No. 5,068,865 (NEC Corporation) or U.S. Pat. No. 4,119,363 (Bell Laboratories). It has been presumed that the same techniques may be employed for the manufacture of lithium niobate devices using a wafer of lithium niobate in which is manufactured an optical modulator or an optical filter. However, reliability trials and tests have shown that a significant number of lithium niobate devices manufactured by conventional semi-conductor techniques have failed very much earlier than would be expected, and typically after only a few hundred hours of operation.
A typical manufacturing cycle for a lithium niobate device comprises the steps of:
production by photolithography of an optical waveguide pattern on a surface of a lithium niobate wafer;
deposition of titanium on the pattern, with a line width in the range of 5-8 .mu.m and a thickness in the region of 700-1100 .ANG.;
diffusion of titanium into the lithium niobate by heating the wafer in a controlled atmosphere, for a period of between 6-12 hours at a temperature of between 1010.degree. C. to 1050.degree. C.;
deposition of a silicon dioxide buffer layer over said surface of the wafer;
annealing of the silicon dioxide buffer layer;
production by photolithography of the required electrode pattern on the buffer layer;
deposition of chrome/gold electrodes on the electrode pattern; and
etching of the buffer layer, from the gaps between the electrodes.
Following the manufacture of the wafer, the component is packaged to complete the device, ready for supply to a user, by the following steps:
mounting the wafer within a metallic container (package);
connecting short lengths of optical fibres ("pigtails") to the optical waveguides and wires to the electrodes, the pigtails and the wires extending through suitable openings in the container and being hermetically sealed thereto;
baking the package and component in a seam-sealer under dry nitrogen, typically for periods of about 4 to 24 hours at temperatures between 60.degree. C. and 125.degree. C.; and
seam-sealing a lid to the container to complete the package.
After a few hundred hours of testing, approximately 50% of devices manufactured as described above have exhibited a phenomenon known as "fast bias drift" (FBD).sub.1 where the bias required to achieve quadrature rapidly increases with time. The time constant of such FBD observed was typically of the order of a few seconds.
It has been reported in the literature that there are certain material effects which can lead to a change in the required bias voltage. The research has however shown that the material effects give rise to time constants of the order of minutes for thermal effects, through to hours for "standard" DC drift phenomena. FBD.sub.1 with a time constant of the order of a few seconds, has not previously been reported in the literature.
Research into the FBD failure has given rise to a partial understanding of the mechanism by which FBD failure of a device can occur. It appears as though lithium migrates from the lithium niobate wafer into the silicon dioxide buffer layer during the annealing stage of the silicon dioxide layer which annealing is performed typically at a temperature in the range of 500-600.degree. C., in an atmosphere of flowing oxygen, for a period of several hours. A corresponding annealing step is well-known in the electronics industry where the densification of a silicon dioxide buffer layer can be employed to improve the electrical properties of a device. However, in a lithium niobate device, the surficial layer of the lithium niobate is damaged by the migration of lithium into the silicon dioxide, the damaged layer thus being depleted of lithium oxide: EQU 3LiNbO.sub.4 .fwdarw.LiNb.sub.3 O.sub.8 +2LiO.sub.2
In an attempt to overcome the above problem, investigations have been made into possible manufacturing methods for producing the silicon dioxide layer which may obviate the annealing step of the known manufacturing process. As a result, it has been established that it is possible to perform a chemical vapour deposition process (CVD process) to deposit the silicon dioxide buffer layer on a lithium niobate wafer, at a temperature significantly lower than would be expected to be possible for such a process and that by operating within this temperature region, no further annealing step at an elevated temperature is required.